Wind energy plants with a horizontal rotor axis, to which the methods of the invention are related, comprise a tower and very heavy, large components attached to the tower top. These large components are the nacelle, the electric generator, the rotor comprising at least one rotor blade and, if any, a gearbox for the rotor.
Wind energy plants of different size, power, and of various types are spreading in ever increasing numbers, in order to produce electric energy from the kinetic energy of the wind. The effectiveness of such a wind energy plant depends, among other factors, on the fact that the wind is present as long and as evenly distributed as possible throughout the year.
It is known that the yields which can be produced by wind energy plants from the wind supply distributed over the year are the larger the higher the wind energy plants can be constructed, as in at larger heights the wind blows faster on an average and in a more laminar fashion. This applies in particular to inland regions, or to hilly or mountainous regions.
In the recent years, the trend is moving towards ever increasing wind energy plants due to economic considerations, with the most widespread type of wind energy plants, the type with a multi-blade rotor having a horizontal axis and being provided on a tower, still having the largest market potential. Above all, the problem of this type of wind energy plant is that its large components increase in size together with the height of the hub; for this reason, installing, disassembling, and above all, the maintenance of such large-scale wind energy plants with hub heights starting from 140 meters are crucial factors for economic efficiency.
In recent years, different tower types have been tested to increase the hub height and take the rotor to the higher atmospheric layers with improved wind availability. Here, in particular hybrid towers comprising a shaft made of concrete and a fitted steel mast where the nacelle is placed, as well as girder masts are to be mentioned.
Usually, the construction of such wind energy plants is carried out in wind parks using mobile large-scale cranes. The highest cranes currently available are crawler cranes by which the large components are lifted to hub heights of 140 meters and more. The crawler cranes or also truck-mounted cranes are used to erect the tower itself, for instance by stacking concrete, steel rings, or concrete or steel segments as prefabricated components on top of each other and connecting them. As an alternative to this, using a climbing formwork is also possible, of course. In the latter scenario, the mobile large-scale crane transports the formwork parts and the concrete to the respective height. Sliding formworks are vertically supplied with concrete and construction steel by using rail-type lifts which exclusively serve for transporting the building materials required for concreting to the momentary tower end of the concrete shaft. Having erected the concrete shaft, the sliding formwork is dismantled and transported to the ground via the rail-type lift.
The daily rents for such large-scale cranes are immense, independently of the fact that only few of these special devices are available in the individual countries and are fully booked due to tight plans of action. The mere construction of a large-scale crawler crane may already take several days. Cranes with a hoisting capacity of 150 tons and a reach of up to 140 meters have assembly periods of roughly three days.
Daily rents of 10.000.00 Euro and more are common. What is more, these large-scale cranes can be operated at low wind speeds at best, so that naturally delays at the building site occur as soon as the weather is less than optimal, as the distance between the standing position and the installation point amounts to more than 140 meters.
For maintenance purposes, in particular in the event of defects on the large components, the large-scale cranes must be rented in the short term, which is not possible at all in many cases because of the low quantities available on the market. Further, the paths leading to the wind energy plants and the area around the wind energy plants must always be kept in good condition or kept cleared for these large-scale cranes in order to facilitate a short-term repair operation with large-scale cranes at all. This circumstance also reduces the income return of the wind energy plants due to the downtimes of the plants until the large-scale cranes are ordered, erected and are operated at suitable weather conditions.
In order to avoid the use of expensive large-scale cranes, growing tower slewing cranes have been used in the past during the construction of towers of wind energy plants. These cranes are capable of setting up themselves starting from a height of approximately 30 meters and are relatively powerful, having a maximum load on the cantilever arm of more than 1000 meter-tons. They can reach heights in almost unlimited fashion, so to speak, but also have enormous daily rents, as they provide a very high performance throughout the entire period of the building procedure. With such growing, large-scale tower slewing cranes, the tower is erected and the large components are placed on the tower top after having erected the tower. Here too, the distance between the last support point and the installation point is 50 meters or more. In the event of maintenance, the entire tower slewing crane has to be ordered, set up, and used again.
Further, a girder mast tower of a wind energy plant comprising a temporarily fitted crane head has already been built. With this crane head and an obliquely running support cable, it was possible to hoist the nacelle to the tower top. The traction cable was deflected on the crane head and guided to a winch fastened to the ground. The crane head was dismounted when the nacelle had been attached. Subsequently, the lighter rotor blades were transported to the hub by using separately standing, mobile large-scale cranes.
In order to carry out any repair of large components in a short time, proposals have already been made to permanently install a crane on the tower top, with which the large components can be exchanged. This proposal has not been able to assert itself, as the cost for such a highly motorized and powerful crane are too high in relation to the number of its applications.
Both the mobile crawler girder mast cranes and the tower slewing cranes are transported to their place of action on dedicated, special low-bed trucks and erected there completely, or in part, with auxiliary cranes. The logistics for setting up a crawler girder crane or tower slewing crane having the required performance is enormous and produces further costs for the transport to and from the site, as well as for erecting and dismantling.
In the case of hub heights of more than 170 meters, free-standing crawler girder cranes are not available at present.